ADP-ribosylation factor domain protein 1 (ARD1) is a member of the ADP- ribosylation factor (ARF) family of guanine nucleotide-binding proteins that differs from other ARFs by the presence of a 46-kDa amino-terminal extension, which acts as a GTPase-activating protein (GAP) for its ARF domain. Similar to ARF GAPs, the GAP domain of ARD1 contains a zinc finger motif and arginine residues that are critical for activity. It differs from other ARF GAPs in its covalent association with the GTP- binding domain and its specificity for the ARF domain of ARD1. ARFs are presumed to play a key role in the formation of intracellular transport vesicles and in their movement from one compartment to another. We report here that ARD1 overexpressed in cells, as a fusion or nonfusion protein, is localized in vesicular structures that are concentrated mainly in the perinuclear region, but are found also throughout the cytosol. Microscopic colocalization and subcellular fractionation studies showed that ARD1 was associated with elements of Golgi and lysosomal structures. ARD1, expressed as a green fluorescent fusion protein, was first detected in association with the Golgi network and subsequently seen localized to lysosomes. Lysosomal and Golgi membranes isolated from human liver by immunoaffinity contained native ARD1, thus localization to these organelles, did not appear to be an artefact of overexpression. These observations suggest that the ARF-related protein ARD1 may play a role in the formation or function of lysosomes and in protein trafficking between Golgi and lysosomes. Continuing work on the ARD1 gene suggests that transcription regulatory elements may reside at a considerable distance from the proximal 5-UTR. - ARF, ARD, GTPase- activating protein, lysosomes, Golgi